4:38 MR. J. F. JACKsox ON [yoL IxxTiii, 



two or three species of small, thick-ribbed, involute Dactyloids 

 are not uncommon. The hifrons layer is separated into two 

 or three minor seams by impersistent planes of erosion, and occa- 

 sionally the ammonites subjacent to these planes are in a similar 

 condition to those at the top of the striatidum layer. Some of 

 the ammonites are embedded in a vertical or highly inclined 

 position, and in one or two instances which came under my obser- 

 vation the ammonite was seen to be truncated by a plane of 

 erosion : a portion having been removed while the remainder con- 

 tinued firmly fixed in the rock, which must have been completely 

 consolidated previous to the erosion. The division between the 

 hifrons Rndfalciferiun layers is often rather obscure, and falsely 

 suggestive of a passage. 



The fed cif em I'll layer is a very hard tough limestone of a 

 yellowish-pink colour, often mottled with red blotches ; occasionally 

 it is rather greenish. Sometimes the rock is fine-grained and 

 compact ; more often it is his?hlv cono-lomeratic, containing 

 broken and rolled Harpocerata and pebbles of a similar limestone. 

 As in the hifrons layer, there are minor layers separated by planes 

 of erosion. Apart from Hcrrpoceras fdciferum. fossils are un- 

 common, and all are very difficult to extract on account of the 

 intense hardness of the stone. 



Tlhe fal ciferuui layer frequently rests directly upon a planed-off 

 surface of ' marlstone," but in places it appears to be transitional 

 from a thin, earthy, ferruginous seam with JPleurotomaricB and 

 other gastropods — presumably the * Pleurotomaria Bed' of Day.^ 



That writer's description is so vague that it is difficult to 

 determine whether he considered the '^ Pleurotomaria Bed" to be a 

 separate layer at the base of the "'Upper Lias' limestones, or merely 

 the highest portion of the ' marlstone." Mr. Buckman has observed 

 an impersistent seam with nests of JPIeurotomaricB at the tojD 

 of the lower or spinafuni layer of 'marlstone.' Avhich he considers 

 to be the true ' Pleurofomaria Bed." I have not noticed any 

 very definite ' JPIeurotomaria. Bed." although JPIeurofomarice are 

 faiidy common in the serraia bed and rare in the main mass of 

 the lower or spinatum layer. It is, perhaps, doubtful whether 

 there is any particular horizon es^^ecially entitled to the term 

 * Pleurofoniaria Bed.' 



At Down Cliff a lenticular deposit of clay intervenes between 

 t\\(d fcdciferum layer and the 'marlstone': but, since no fossils 

 were obtained, it is impossible to say anything as to the exact 

 date of this clay-band. The occm-rence of a lenticle of clay 

 within the Junction-Bed is notewoiiihy. 



The spincdum layer or 'marlstone,' when fully developed, is 

 divisible into two seams. The upper, Mr. Buckman"s serrata bed, 

 is usuallv a fine-grained oolitic limestone with a rich assemblao-e 

 of well-preserved fossils : occasionally it is conglomeratic. The 



' ' On the Middle & Upper Lias of the Dorsetshire Coast ' Q. J. G. S. 

 Tol. xix (1863) p. 288. 



